Discussion of Prior Art
The criteria for a trench-box strut are as follows: Firstly, the strut is required to apply force to a trench-box panel so as to press the panel against a wall of an excavation in order to shore up the wall. The strut is also required to pull the panel away from the wall when it is desired to remove the trench-box from the excavation. The strut is not required to position the panel with any particular accuracy, nor does the adjustment of the strut require fine control of the strut. However, considerable robustness and strength is required of the strut. If the strut is a screw-type strut, which is usual, if not universal, the screw mechanism is required to be manually operable in both directions, that is, both when extending the strut to press the panel against the wall and when retracting the strut to release the trench-box, and is required to maintain its extension under external applied force when in use. Hence, the screw must be easy enough to turn manually in either direction. It is preferred for only a few turns to be required to retract the strut or extend the strut as desired.
It is common practice to machine screw threads to achieve dimensional tolerances which are acceptable for the particular uses required of the screw threads. However, any machining of a screw thread is a relatively costly operation.
One particular size of standard "Acme" thread has a pitch of 12 mm, giving a feed of 12 mm or 24 mm for a single-start or double-start thread accordingly. This is not an ideal pitch for every application.